PHP is a serverside language, which means, it needs a server to run it’s code ( PHP Code That is)
But when you just use html code in a php file, it’ll be interpreted as html code (php code is wrapped into <?php ..code.. ?> tags, so when you have php code outside these tags, it’ll be interpreted as html, not php)

It doesn’t really matter whether you save it as a .html or .php, but when you start adding php code to an html file, it probably won’t work

I thought about saving files in .php as well, because there’s no real harm when it’s done, but when your buyers want to implement php, they dont have to change the filename.
It may however confuse your buyer, in case they’re only familliar with .html files.